1. Field of the Invention
The present invention generally relates to an apparatus for examining an object by using ultrasonic beams, and particularly to an apparatus in which a sectional area in the object is scanned by the ultrasonic beams and ultrasonic echo signals derived from reflections by boundaries and discontinuities therein are processed in an image processing method.
2. Description of the Prior Art
In general, such an apparatus is essentially comprised of an ultrasonic transducer having a plurality of ultrasonic transducer elements in an array alignment, a scanning unit which excites those ultrasonic transducer elements to generate a plurality of ultrasonic beams and which then scans a plurality of those ultrasonic beams in a sector form so as to obtain echo signal data of the object, a frame memory unit in which the echo signal data is temporarily stored in relation to the ultrasonic beams, and a TV monitor on which a sector-scanned tomographic image of the object is displayed in a real time.
The above-identified sector-scanning type ultrasonic imaging apparatus is known from, e.g., U.S. Pat. No. 4,368,643 issued on Jan. 18, 1983 to Tachita, and U.S. Pat. No. 4,310,907 issued on Jan. 12, 1982 to Tachita.
In such a sector scanning type ultrasonic imaging apparatus, there are the following problems. Since the echo signal data is stored in the given memory position (i.e., pixel) of the frame memory corresponding to the ultrasonic scanning line, the pixelizing, or digitizing precision on the ultrasonic scanning lines in a direction perpendicular to the television scanning direction, i.e., the horizontal scanning direction becomes vry high. On the other hand, the quantizing precision is kept constant, so that a false contour appears in the displayed image.
In other words, in the frame memory, a screen of TV monitor is divided in a matrix form to allocate the memory address to its corresponding pixel, and the column direction of the matrix is coincident with the television scanning direction, i.e., the horizontal scanning direction and also the row direction of the matrix is coincident with the vertical scanning direction, or a direction perpendicular to the longitudinal axis of the array of the ultrasonic transducer elements. The ultrasonic scanning lines are directed from the upper side toward the lower side of a TV screen at angles corresponding to the travelling directions thereof. Namely the echo signal data is stored in the pixels of the frame memory along the imaginarily-drawn lines in such a manner that the signal data on a depth of the object is stored. Accordingly, as the directions of the ultrasonic scanning lines, i.e., those of the imaginarily-drawn lines, are coincident with the orientation of the pixels, the pixelizing precision becomes high with respect to the direction perpendicular to the television scanning direction. However, the directions of the ultrasonic scanning lines are not sufficiently coincident with the orientation of the pixels, so that a substitution pixel is selected to store the echo signal data. This pixel is located adjacent to the pixel which the ultrasonic scanning line originally intersects (i.e., the simple pixelizing operation, or simple digitizing operation).
As a result, when such a frame memory is read out in synchronism with the television horizontal scanning, the readout signal is superimposed with the sync pulses (i.e., the composite video signal), and the resultant video signal is displayed on the TV monitor, the ultrasonic scanning lines are not displayed as straight lines in the any directions except the television vertical scanning direction and the diagonal of the pixels.
In general, the brightness information data of the ultrasonic diagnostic apparatus is processed by a bit length of approximately 6 bits in view of the grey scale of the TV monitor and also the processing speed of the image data. Accordingly the same sampling precision is realized in any position of the ultrasonic scanning line. That is, available gradation is kept constant. On the other hand, since it is known that a smoothing image can be recognized if a balance is maintained between the pixelizing precision and quantizing precision, the following drawback will occur. In such a digitized image the quantizing precision of which is kept constant, but only the pixelizing precision of which becomes high in the certain position of the scanning line, a false contour such as a contour line may occur therein.
As previously described, in the sector scanning type ultrasonic examination apparatus, the image data is temporarily stored in the frame memory and then is read out in conformity with the television scanning system so as to display it on the TV monitor. Consequently, as the directions of the ultrasonic scanning lines change in a shape of a sector, the pixelizing precision varies in accordance with the scanning line positions, especially the pixelizing precision for the lines near a center of the sector scanning region which becomes too high in comparison with that for other scanning line positions. As a result, the above-described false contour occurs.
Moreoever, this false contour may cause not only vague images, but also errors in diagnosis. Accordingly, an improvement is strongly required in this field.
It is therefore an object of the present invention to mitigate such drawbacks and to provide an apparatus for examining an object by using ultrasonic beams, in which a false contour can be prevented without degrading the image quality, the false contour being caused when the quantizing precision is kept constant and only the pixelizing, or digitizing precision changes in accordance with the positions of the ultrasonic scanning lines.